Dry insulated transformer

ABSTRACT

This disclosure includes a high-voltage dry-insulated transformer and to the method of constructing the transformer. The primary coil is a multiple-turn, multiple-layer winding constructed by alternately winding a coil layer and then insulation sheets or tape on a winding form. The insulation sheet is a nonwoven glass filament cloth which is impregnated with a semicured epoxy resin. A plurality of insulating cloth layers are wrapped about the primary coil to define the necessary intercoil insulation. The secondary coil is then similarly formed. The interlayer and intercoil insulating cloth extends beyond the axial end faces of the coils and the space is filled by wrapping of a corresponding tape between the two extended layers. The cloth is applied under tension and heat and the exterior of each layer is rolled to exclude air and cause the resin to flow into the voids and crevices. The exterior is wrapped with a shrinktype Mylar and the ends of the assembly are capped to confine the insulating resin during the curing and processing cycle, the Mylar acts as a positive force during curing to cause the resin to flow into all voids in the coil. The Mylar is removed after the coil has been cured.

United States Patent 3,297,970 1/1967 J0nes..;

Continuation of application Ser. No. 780,335, Dec. 2, 1968, nowabandoned. This application Aug. 26, 1970, Ser. No. 67,288

, DRY INSULATED TRANSFORMER 7 Claims, 7 Drawing Figs.

US. (I 336/182, 29/605, 264/272, 336/96, 336/205, 336/206 Int. Cl H01]27/32 Field of Search 338/205,

References Cited UNITED STATES PATENTS Primary ExaminerE. A. GoldbergABSTRACT: This disclosure includes a high-voltage dry-insulatedtransformer and to the method of constructing the transformer. Theprimary coil is a multiple-tum, multiple-layer winding constructed byalternately winding a coil layer and then insulation sheets or tape on awinding form. The insulation sheet is a nonwoven glass filament clothwhich is impregnated with a semicured epoxy resin. A plurality ofinsulating cloth layers are wrapped about the primary coil to define thenecessary intercoil insulation. The secondary coil is then similarlyformed. The interlayer and intercoil insulating cloth extends beyond theaxial end faces of the coils and the space is filled by wrapping of acorresponding tape between the two extended layers. The cloth is appliedunder tension and heat and the exterior of each layer is rolled toexclude air and cause the resin to flow into the voids and crevices.Theexterior is wrapped with a shrink-type Mylar and the ends of theassembly are capped to confine the insulating resin during the curingand processing cycle, the Mylar acts as a positive force during curingto cause the resin to flow into all voids in the coil. The Mylar isremoved after the coil has been cured.

, l DRYJINSULATED TRANSFORMER 'This case is a continuation ofSer. No.780,335, filed Dec. 2,

'-l,968,'now abandoned.

BACKGROUND OFTHE INVENTION Thisir'iventionrelates to, an inductivecoiland the method of making the'coil and more particularly to ahigh-voltage dry-insulated inductive coil unitof a high-voltagetransformer.

I-Iigh-ivOItageJcoiIs'and transfonners require special insulatingtechniquesbecause of the detrimental effects arising in the presenceofair, -moisture,.as well as other foreign and contaminating 'materialinthe operating environment. Although oil -immersed coils provide-certainoperating advantages, dryinsulated 'and encapsulated'transformer andother inductive coils-havesubstantiabadvantages from the standpoint ofhandling and-the like. However, dry-type-insulation must be speciallyformed andspecialconsiderations taken to avoid the adverse effectsassociated withCorona. Void orforeign material in thecoilinsulationtraps air within the insulation which is subject ,toa voltagedifference; If thevoltage gradient is sufficiently high, actualionizationof the air and sparkover may occur. This' results in rapiddeterioration of the insulating characteristic of the insulationandeventually results in a disruption'or breakdown thereof. Somehigh-voltage transformers and other inductive coils of the-dry-typeinsulation have been formed by encapsulating or potting of the coil witha suitable resinafter the coil iswound. Such coils, however,

have been found to have severe limitations both aste the physical sizeand theuvoltage applications because of entrapment of air within theinsulation.

SUMMARY The present invention isp articularly directed to anencapsulated high-voltage induction'coil provided with an encapsulatingdry insulation and to the method of making such coil prac- .are disposedin'superimposed relationship such as in a trans- .former toform'thenecessary intercoil relationship. Free resin may be applied .tothe-insulating material during winding if a larger-'amountofiresin isrequired for a particular application. The interlayer and intercoil;insulating cloth preferably extends beyond the axialend faces of thecoil to the desired end insulating depth. Thespace at eachend betweeninsulating layers is filled with a corresponding layer insulation bycontinuous wrapping between the two layers. The various insulating clothsheetsarepreferably applied under tension and the exterior of each layerpreferablyrolled or otherwise worked to excludeair. The insulatingclothis also warmed slightly during the winding and workingto cause the resinon the insulation material to more readily flow into the voids andcrevices between the coil turns. After the formation of the coil withthe interposed insulation, it is subjected to a curing and processingcycle including the heating under pressure and vacuum to cure the resinand remove all air.

In a preferred and novel construction after the winding of a coil withinsulation material as described above, the exterior is preferablywrapped with a heat shrink type material such as a shrink-type Mylar.The endfaces of the coil are capped to confine the insulating resinduring the curing and processing cycle.

The heat shrink outer layer contracts during the curing process andestablishes a positive force causing the resin to flow into all of thevoids arid crevices and more surely excludes air from the insulationwhereby a void and air free solid insulation about the windings isobtained. After the curing the Mylar wrapping is removed.

1 teaching of the present invention;

In the formation of a transformer the stepspreferably include thesequential forming, of the in'nei windingv including the curing andprocessing of the insulation. The'major intercoil insulation is thenapplied, [cured and processed,

Thereafter, the outer winding is wound with thedesired. insulation in acorresponding manner and is then curedyand processed in thesame-procedure to set the insulating material of the second winding andcomplete the transformer, If the transformer is not too .lar geatheiseveralwindings'and .the

major intercoil insulation may be wound in a single sequence with thecuring the processing simultaneously provided for the a completed coil.

BRIEF DESCRIPTION OF THE; DRAWINGS FIG. 1 is a plan view of atransformer withparts-broken away to show details of:constructiori; 1 w

FIG. 2 is a side elevational view of the transformer shownjin.

FIG. 5 is a diagrammatic illustration of a preferred method for applyingthe windings and the insulation in theformation of a coil;

FIG. 6 is a side elevational view ot1FIG. 5; and I FIG. 7 is adiagrammatic illustration-showing the coil placed inacuring oven. 7 t

DESCRIPTIONOFA PREFERRED s reoomiem Referringto the drawingsand-particularly to FIG. 1,: the present invention is illustrated inconnection with an ahnularr or 'doughnut-shaped transformer having'aninner primary winding I wound on a tubular winding-form 2, The primarywinding 1 is a multipledayer and multiple-tum winding having a pluralityof primarytaps and end connectors 3. Superim posed in axially stackedrelation about the primarywinding .l' is a delta secondary winding4'anda wye secondary winding" The connectors 6 for winding 4 ;projectlaterally or'radially from the periphery of the transformer and similarconnectors 7 project radially from the lower-portion'of the coil.- Thepri-j mary winding 1 is separated fromtliesecondary windings 4.

and S by a relatively heavy layerot major insulation 8."The

ends of the transformer 1 and the secondary windings 4 and 5 areseparated by aheavy layer of insulation 9 iand 'l0. Additionally, theindividual layers ofcoil tums-are separated from each other byinterlayer insulation 11 of a varying "thickness and the periphery issealed by a layer of insulation 12 The insulation 8 through 12 is fusedto form a solid; integral mass which completely encapsulates the coilsor windings.

Referringparticularly to FIG. 4, the diagrammatic illustration of thewinding layout is shown for purposes of illustrating the windingconstruction of the transformer and the method of forming suchtransformer. I

The several windings or coils are wound about the centra tubular windingform 2 which corresponds in length to the completed transformer. One ormorelayers of an'=insulatit1g;

sheet 13 is wound about the form 2, The insulating sheet 13, and all thesubsequent tapes and sheets" of insulating described, are a nonwovencloth which is pr'eimpregnated with a semicured epoxy resin. The resincontent was approximately 40 percent by weight. Such an insulating tapeistrianufa'ctured and sold under the trademark ScotchplyVb'yMinnesotaMining and Mariufacturing Company. In an actual construction, asingle layer of insulating tape'l3 corresponding to the heights of theform 2 and of a 20mill thickness was employed. The first primary coillayer 14 is wound upon the tape covered form 2 as a plurality ofimmediatelyadjacent convolutions.

Referring particularly to FIG. 2 in an actual construction, each turnwas fonned of a pair of immediately adjacent square portion .of the:transformer.

conductors and 16 for convenience and ease of winding. The conductors 15and 16 are disposed in immediately adjacent axial side by siderelationship and spirally wound upon the tape 13; to form themultiple-turn layer 14. The initial turn is spaced downwardly from theupper end of the coil form 2, shown to'the left in FIG. 4, and the lastturn is correspondingly spaced from the lower end of the coil form 2.

The coil turns of layer 14 may be individually wrapped with insulation,not shown, with the total length dimension generally corresponding toall of the other layers as hereinafter described After the winding ofthe first multiple layer 14 is completed, the end spaces adjacent theopposite end coil turns of the first coil 14 are filled with aninsulating tape 17. For example, in an actual construction, the coilform 2 extended approximately 1 inch beyond the end turns of the coillayer 14. The space was filled with a suitable tape wrapped around bothends.

Two superimposed sheets or layers 18 of insulating cloth are thenapplied over the first coil layer 14. As previously described withrespect to FIGS. 1 and 2, the first coil layer 14 of the primary istapped to provide the several primary tap connections 3. In theillustrated embodiment of the invention, the insulating sheets 18include appropriate axially spaced openings 19 in alignment with theproper coil turn to produce the desired voltage tap. The severalopenings 19 are also circumferentially spaced to permit interconnectionof the several taps 3. The taps 3 are shown superimposed in FIG. 4 inaccordance with the usual convention.

The connectors 3 may be in any desired manner such as a flat contactstrip extending downwardly adjacent the outer surface of the insulatingsheets 18, with the intermost end brazed or otherwise connected to theadjacent coil turns of layer 14. The connections 3 are preferably woundwith a plurality of layers of an insulating tape.

A second layer including a pair of insulating sheets 24, generallycorresponding to sheets 18, is wrapped about the coil and tappedconnections. The second coil layer is then reverse wound on the firstcoil layer 14, that is from the lower end of the coil, shown to theright in FIG. 4, upwardly with the uppermost turn terminating inalignment with the uppermost turn of the first coil layer 14. Thewinding turns may be formed from an integral conductor with theinterconnection 26 extending through a suitable edge slot and openings27 provided in the insulating sheets. In the transformer constructed,the first coil layer included turns with the turns gapped to extend thewinding over 22 inches of the 24-inch length of tube form 2. Generally,the wrapping of the individual turn of layer 14 with insulation 15accounted for the additional space of such turns. The second coil layer25 and all subsequent coil layers were wound with coil turns, such thatthey constituted essentially a solid copper conductor throughout the 22winding inches.

After the winding of the second coil layer 25, the end spaces are filledwith the suitable size insulating tape to extend the insulationoutwardly to the end face of the form 2. The second coil layer 25 iscovered with insulating sheets 28 similar to that for layer 14.Additional coil layers 29 and a final coil layer 30 are successivelysimilarly applied and wrapped with insulating sheets with the outer endspaces being filled with the filler insulating tape. In the illustratedembodiment of the invention, a total of six layers are shown with theoutermost layer having its end terminating in the upper plane of thetransformer and having the lead 3 extend outwardly to define theopposite end of the coil connection.

In preferred construction, the last five turns of the primary coil layer30 are individually wrapped with a tape similar to the wrapping-of theinitial turns of the first coil layer 14.

Although the winding of the coil turns and the applying of theinsulating tape and sheets may be in any suitable manner, a preferredmethod is diagrammatically shown in FIGS. 5 and 6. Each insulating sheetis applied through a suitable tension means 31 to securely wrap eachlayer of insulation upon the adjacentcoil layer. Additionally, a heatsource 32, such as a suitable heat lamp, is applied to soften the resinin the insulating tape and sheets and thereby transform the resin into a'flowable state such that the resin will tend to flow into and fill thevoids and crevices. Additionally, each layer of insulation is rolled orworked onto the adjacent coil layer as by a roller 33 as it is appliedto remove all significant amount of air from between the layers and theadjacent member.

After the winding of the final coil layer 30, the assembly is removedfrom the winding apparatus and one or more layers of a heat shrink tapeor sheet 34 such as Mylar is wrapped about the outer peripheral surface,as shown in FIG. 7.

The Mylar may be a 2-mill film type commercially available under thetrademark Scotch-lite.

End caps 35 and 36 are secured to the opposite end faces of the primarywinding or coil in clamping engagement with the end faces of the coil toconfine the insulating resin within the coil. The Mylar-wrapped andcapped assembly is disposed within a suitable curing apparatus 37 andsubjected to an appropriate curing cycle including heat, pressure andvacuum in accordance with well-known procedure. The heating of theassembly causes the heat shrink Mylar to contract and thereby forces theresin to flow into all voids and crevices, resulting in a solid,void-free insulating mass. After completion of the curing process theend caps 35 and 36 and the Mylar cover 34 are removed.

The cured insulated primary coil 1 is reassembled with the wrappingapparatus.

Major intercoil insulation 8 is then applied to the outer periphery ofthe cured primary. In the practical construction, 25 layers of 20-millScotchply cloth sheet 38 wrapped about the primary to defineapproximately 56-inch thick major insulation.

The several layers 38 are wrapped onto each other as diagrammaticallyshown in FIGS. 5 and 6 to maintain tension on the individual layerswhile they are simultaneously subjected to heat of source 32 and aroller working of roll 33 to remove air and the like.

The assembly is then again removed and placed in the curing apparatus 37and the insulation layers 38 subjected to the same curing process todefine a solid mass of major insulation 8 which in turn blends and bondsto the outer periphery of the primary coil 1.

The cured assembly is then again applied to the wrapping apparatus toreceive the secondary windings 4 and 5, the coil turns of which aredisposed in stacked relation as shown in FIGS. 2 and 4. I

The delta winding 4 is started at the top with the initial turngenerally aligned with the top turns of the several coil layers of theprimary winding or coil 1. Each coil turn is formed from a pair ofconductors generally similar to the primary winding 1. The first andlast three turns of the delta winding are preferably individuallywrapped with insulating tape which may be similar to the individualwrapping of the individual turns of the first primary coil layer 14. Asingle layer of tape was applied between each of the coil layers.

The delta winding 4 of the transformer constructed spanned 50 percent ofthe depth of the transformer. Each coil layer 39 was wound with l6 turnsand six coil layers 39 were provided. Each coil layer 39 wasconcentrically wound with the opposite end turns spaced respectivelyfrom the outer end or top face of the finished transformer and from thecenter of the transformer and thus from the wye winding 5. The coillayers 39 were separated by a sheet of insulation 40 and the end spacesto the opposite ends of the coil turns were similarly filled withsuitable insulation tape 41, as shown in FIGS. 2 and 3 and morespecifically described with respect to the primary.

The delta winding spanned approximately 9% inches and was spacedinwardly from the outer face approximately 1%- inch and from the centerof the assembly approximately onehalf inch. A lie-inch 20-mill Scotchplytape 41 was applied to the outboard end of the delta winding and a 178-inch 20-mill Scotchply tape 41 was applied to the inboard end of thewinding to fill the adjacent space and maintain a continuous insulatingtape filler to the opposite end of the winding.

The wye secondary winding 5 is similarly applied to the.

lower half of the transformer. The wye winding includes three coillayers 42. The diagrammatic illustration of FIG. 4 shows a substantialvoid between the outermost layer and the outer surface of thetransformer. In fact, each coil layer includes a double radial layer ofconductors 43, as shown in FIGS. 2 and 3, such that each coil turnincludes four conductors arranged in four quadrants to define a squaremulticonductor for each turn. Each coil layer 42 will generallycorrespond to two of the coil layers of the delta winding and theradially extension will correspond to that of the deltawinding.

Additionally, the coil layers 42 of the wye winding 5 are separated by adouble layer of insulation 44. In the actual transformer'construction,the winding was spaced from the center of the assembly by approximatelyone-half inch and spanned approximately inches thereby spacing thelowermost turns from the outer face by approximately l'z-inch. The endspaces were filled by ye-inch by -mill tape and a 1%- inch by 20-milltape 46, similar to the delta winding.

After the formation of the delta winding 4 and the wye winding 5 adouble layer of insulating sheets 47 of a full depth is applied aboutthe secondary winding.

Stress cone insulation 49, as shown in FIG. 2, may be applied to the topand bottom by wrapping a plurality of layers of insulating tape 8 aboutthe upper and lowermost ends of the coil. For example, the upper andlower 5 inches of the peripheral surface of the 24-inch high transformerwhich has been described herein was formed with a stress cone. Thestress cone was formed by 2-inch by 20-mill tape with an overlap wrappedto define the thickness to the outermost ends.

Three layers of heat shrink Mylar are then applied about the completedwindings with the connecting end of the several windings extendingoutwardly through the tape insulation and the heat shrink Mylar. Theassembly is removed from the winding apparatus and end caps 35 appliedto again confine the insulating resin.

The assembly is placed in the curing apparatus 37 and is subjected tothe heat, pressure and vacuum in accordance with known procedures tocompletely cure the resin. Once again, the shrinking of the outer Mylartape causes the resin to flow into all the voids and crevices andthereby insures a solid and continuous insulation. The stress cone areasare then ground to a suitable radius to complete the transformer.

The several primary taps 3 and secondary connectors 6 and 7 may bewrapped with a suitable tape or other insulating material 51 to defineintegral stress cones for removing the high-voltage stress from thepoint of termination and thereby minimizing or eliminating corona on theexternal surface.

The transformer constructed in accordance with the present invention hasbeen found to provide a highly reliable and ef' fective high-voltageinsulation.

Although the curing step for'the primary and secondary windings as wellas the major insulation has been described as individual sequentialsteps, they may be simultaneously provided if the construction and thecuring apparatus permit.

A transformer constructed in accordance with the illustrated embodimentof the invention has advantageously been employed in a high-voltagedirect current power supply for a plasma arc process. A three-phasetransformer was employed with each of the phases provided by atransformer as shown in the drawings.

The particular embodiment of the invention set forth above including theparticulars of the number of turns, spacing of coils, insulating layersand the like are for the purpose of clearly explaining one structure.Such detail as spacing,

I. An inductive coil means to be formed into a totally encapsulated coilcomprising, a plurality of spaced coil layers, insulation disposedbetween said layers and including at least one layer of a cloth materialimpregnated with an uncured resin, said cloth material projectingaxially substantially of said coil layers, and means responsive to heatfor compressing said coil layers by shrinking into tight engagementtherewith to cause the resin in the insulation to flow into the voidsand crevices of the coil thereby forming a solid insulating mass.

2. The inductive coil means of claim 1 wherein said insulation istotally of a nonwoven cloth material impregnated with an uncured resinand includes a layer of cloth material between each coil layer, saidcloth material projecting axially outwardly of said coil layers, and aplurality of superimposed layers of said cloth material filling the endspaces aligned with said coil layers. 1

3. The inductive coil means of claim 1 including a spaced multilayersecondary winding having a plurality of spaced coil layers, and aninterlayer of cloth material impregnated with an uncured resin beingdisposed between each layer of said secondary winding, means responsiveto heat for compressing said secondary winding by shrinking into tightengagement therewith to cause the resin to flow into the voids andcrevices forming a solid insulating mass, and a plurality of layers of acloth material disposed between said windings to form an intercoilinsulation, and means responsive to heat for compressing said intercoilinsulation by shrinking into tight engagement therewith to cause theresin in the intercoil insulation to flow into the voids and crevicesforming a solid insulating mass.

4. The inductive coil means of claim 3, wherein the interlayer andintercoil cloth material of said secondary winding projects axiallyoutward to define end insulation, and a plurality of layers of clothmaterial impregnated with an uncured resin concentrically wrappedbetween said projecting cloth material to complete the end insulation.

5. The inductive coil means of claim 1 having outwardly extendingterminal means and integral stress cones extending outwardly about saidterminals from the outermost layers of insulation to reduce dielectricstress concentration.

6. An inductive coil means to be formed into a totally encapsulated coilcomprising, a plurality of spaced coil layers, and insulation disposedbetween said layers including a layer of a cloth material impregnatedwith an uncured resin between each coil layer, and a heat-shrinkablecover responsive to the flow temperature of the resin covering said coillayers and insulation whereby heating of said coil shrinks said coverand forces the resin to flow into the voids and crevices in the coillayers to create a solid and continuous insulation.

7. The inductive coil means of claim 1 wherein said insulation istotally of a nonwoven cloth material impregnated with an uncured resinand includes a layer of said cloth material between each coil layer.

1. An inductive coil means to be formed into a totally encapsulated coilcomprising, a plurality of spaced coil layers, insulation disposedbetween said layers and including at least one layer of a cloth materialimpregnated with an uncured resin, said cloth material projectingaxially substantially of said coil layers, and means responsive to heatfor compressing said coil layers by shrinking into tight engagementtherewith to cause the resin in the insulation to flow into the voidsand crevices of the coil thereby forming a solid insulating mass.
 2. Theinductive coil means of claim 1 wherein said insulation is totally of anonwoven cloth material impregnated with an uncured resin and includes alayer of cloth material between each coil layer, said cloth materialprojecting axially outwardly of said coil layers, and a plurality ofsuperimposed layers of said cloth material filling the end spacesaligned with said coil layers.
 3. The inductive coil means of claim 1including a spaced multilayer secondary winding having a pluralIty ofspaced coil layers, and an interlayer of cloth material impregnated withan uncured resin being disposed between each layer of said secondarywinding, means responsive to heat for compressing said secondary windingby shrinking into tight engagement therewith to cause the resin to flowinto the voids and crevices forming a solid insulating mass, and aplurality of layers of a cloth material disposed between said windingsto form an intercoil insulation, and means responsive to heat forcompressing said intercoil insulation by shrinking into tight engagementtherewith to cause the resin in the intercoil insulation to flow intothe voids and crevices forming a solid insulating mass.
 4. The inductivecoil means of claim 3, wherein the interlayer and intercoil clothmaterial of said secondary winding projects axially outward to defineend insulation, and a plurality of layers of cloth material impregnatedwith an uncured resin concentrically wrapped between said projectingcloth material to complete the end insulation.
 5. The inductive coilmeans of claim 1 having outwardly extending terminal means and integralstress cones extending outwardly about said terminals from the outermostlayers of insulation to reduce dielectric stress concentration.
 6. Aninductive coil means to be formed into a totally encapsulated coilcomprising, a plurality of spaced coil layers, and insulation disposedbetween said layers including a layer of a cloth material impregnatedwith an uncured resin between each coil layer, and a heat-shrinkablecover responsive to the flow temperature of the resin covering said coillayers and insulation whereby heating of said coil shrinks said coverand forces the resin to flow into the voids and crevices in the coillayers to create a solid and continuous insulation.
 7. The inductivecoil means of claim 1 wherein said insulation is totally of a nonwovencloth material impregnated with an uncured resin and includes a layer ofsaid cloth material between each coil layer.